The present invention relates to improvements in the performance characteristics of components having a natural rubber base which is susceptible to degradation caused by oxidation, ozone attack or ultraviolet radiation, especially under extreme heat, by coating exposed surfaces of the natural rubber based product with one or more elastomers having a high resistance towards oxidation under excessive heat.
Because good durability is a major consideration for rubber engineering components, products formed from natural rubber are fairly common. Components formed from natural rubber tend to have excellent resilience, high tensile strength, high resistance to tearing, good flexibility at low temperatures and excellent mechanical fatigue resistance due to the strain crystallization ability of the material. However, natural rubber based materials tend to have relatively poor resistance toward oxidation, ozone attack and ultraviolet radiation, each of which can cause significant deterioration in the mechanical strength of natural rubber based components. Further, oxidation of natural rubber tends to occur much faster at higher temperatures than would occur at ambient temperatures, thus, the useful life of natural rubber based components used under heated conditions tends to be relatively short.
In general, there are considered to be two main causes for the degradation of natural rubber based components at temperatures in excess of approximately 220.degree. F. The first cause is generally associated with the accelerated reaction of C.dbd.C bonds with oxygen and ozone at high temperatures. The other cause relates to the breakdown of polysulfide linkages in vulcanized compounds under excessive heat.
Due to the inherent problems associated with utilizing natural rubber based components in high temperature environments, attention has been focused on using more thermally and oxdatively stable elastomers such as Neoprene, ethylene-propylene terepolymers such as EPDM, hydrogenated nitrile-butadiene rubber and polypropylene oxide, among others. Elastomeric materials such as these have little C.dbd.C unsaturation in their polymer chains are less reactive towards oxygen, thereby making them much more stable with respect to high temperature applications, especially in the presence of oxygen and/or ozone. Unfortunately, however, these materials generally fail to offer the resistance against mechanical forces that arise due to their lack of strain crystallization. Further, such materials are generally more expensive than components formed from natural rubber, thus, making them less attractive for high scale production.
While attempts have apparently been made to solve certain problems relating to polymeric substrates in general which are subject to degradation caused by oxygen, ozone and ultraviolet radiation heretofore, the special circumstances involved in high temperature and high mechanical strength requirement applications have not been addressed. For example, U.S. Pat. No. 5,314,741 entitled "Rubber Article Having Protective Coating" which issued on May 24, 1994, to Robert et al. relates to polymeric articles which are coated with hydrogenated synthetic rubbers or polymers obtained by hydrogenating an unsaturated polymer which is a polymer of 1,3-butadiene and optionally one or more monoethylenically unsaturated polymers. The hydrogenated unsaturated polymer or monoethylenically unsaturated polymer based coating is applied as an uncured highly saturated polymer latex which is dried and cured in place on the substrate.
In contrast to other known coatings for polymeric materials which are directed to corrosion resistance or protection against degradation caused by oxidation or ultraviolet light under substantially ambient conditions, it is the primary object of the present invention to provide natural rubber based components which are protected from degradation caused by oxidation, ozone attack or ultraviolet radiation, under extreme heat, by providing the component with a coating of one or more thermally and oxidatively stable elastomers.
It is another object of the present invention to provide coating compositions applicable over natural rubber based materials which are resistant to heat and oxidation, have relatively low gas permeabilities and are durable in nature.
It is another object of the present invention to provide methods for preparing components having a natural rubber base and a coating of one or more thermally and oxidatively stable elastomers.
The present invention relates to natural rubber based components and components which are coated with a natural rubber based material, wherein the components are coated with at least one elastomer.
By applying one or more layers of the elastomeric coatings over a natural rubber based material, the natural rubber based material has an enhanced resistance toward oxidation under excessive heat. Further, the C.dbd.C bonds of the natural rubber based material are protected from oxygen and ozone attacks, thereby allowing the natural rubber based material to retain its strain crystallization property. Thus, by coating the natural rubber based material with one of the elastomers, the natural rubber based component or component coated with a natural rubber based material has enhanced resistance toward thermal oxidative aging.
The present invention, while useful under ambient conditions, is particularly useful in environments which are subject to excessive heat. For example, one application of the present invention relates to the manufacture and use of automotive vehicle engine mounts. With the advent of smaller automobiles and front wheel drive systems, engine compartment sizes have generally decreased. This, coupled with the development of more powerful engines and the use of more thermally conductive metals for engine components, has resulted in increased temperatures occurring under the hood. The natural rubber based components and components coated with a natural rubber based material under the present invention are, therefore, ideally suited for engine mounts and gaskets which require a minimal average useful life of at least 3-5 years as is generally required by the automobile manufacturing industry.
Still other applications for the natural rubber based components and components coated with a natural rubber based material which are further coated with an elastomeric coating in accordance with the teachings of the present invention will become apparent to those skilled in the art upon review of the description of the invention and claims, taken in conjunction with the following drawings.